This invention relates to electronic article surveillance (EAS) systems. These systems are often used to detect shoplifters or otherwise monitor the movements of tagged articles. The EAS systems of the type to which the subject invention is directed, are generally employed to detect the presence of a magnetic marker in a magnetic field. Such systems thus include a device, for example a generating coil, for generating the magnetic field, and another device, for example a receiving coil, for detecting signals generated when a marker is passed through the field.
Important considerations in the design of EAS systems include, for example, power consummation of the system, cost of the system, signal-to-noise ratio of the received signal, and sensitivity to a marker in the detection field. Factors which may influence the signal-to-noise ratio and sensitivity to a marker in the field may include coupling of the generated magnetic field and/or marker generated fields in the receiver, metallic objects in the vicinity of the EAS system, spurious emissions, and electromagnetic interference. Improving the signal-to-noise ratio and sensitivity to a marker in the field generally involves increased power consumption of the EAS system transmitter and/or receiver circuitry.
Prior disclosed EAS systems have utilized pulsed transmitter signals and corresponding detection of the signal generated by a marker located in the magnetic field produced by the transmitter. For example, both electromagnetic (EM) and radio frequency (RF) EAS systems have been disclosed which utilized a pulsed transmitter signal. These systems have received, during the "off" cycle of the transmitter signal, the signal generated by a marker due to the marker's response to the transmitter signal generated during the "on" cycle of the transmitter. In these systems, the pulsing of the transmitter resulted in somewhat better signal to noise ratios. However, no attempt was made to detect modulation as an indication of the presence of a marker.
In addition, prior EAS systems, for example, in U.S. Pat. Nos. 4,710,752 and 5,005,001, have also been disclosed which create modulation via generation of two overlapping magnetic fields having different frequencies. In these systems, the amplitudes of sidebands of the harmonics, generated by a marker, were compared as an indication of marker presence. However, these systems did not attempt to detect modulation as an indication of the presence of a marker.